Immunoelectron microscopic demonstration of the membrane proteases aminopeptidase N/CD13 and dipeptidyl peptidase IV/CD26 in normal and neoplastic renal parenchymal tissues and cells

Aminopeptidase N (APN, CD13) and dipeptidyl peptidase IV (DPP IV, CD26) are transmembrane ectoenzymes occurring in a wide variety of cells. They are i nvolved in tumour cell invasion and the formation of metastases. A basis fo r further information about these enzymes is the exact ultrastructural loca lization in normal and malignant cells. In this paper, we demonstrate the p recise subcellular localization of the membrane peptidases APN and DPP TV o n the cell surfaces in renal tissues, renal cell carcinoma, cultured renal parenchymal cells and cultured renal carcinoma cells. Using cryo-ultramicro tomy of weakly fixed tissues and cells in combination with indirect immunog old labelling, both membrane peptidases were detectable on the external cel l surfaces. They showed different ultrastructural expression patterns. Both membrane peptidases were abundantly labelled on the external cell surfaces of human kidney proximal tubular cells. The expression pattern of APN/CD13 and DPPIV/CD26 in single labelling was confirmed by a successive double la belling technique. The immunolabelling of CD13 on cultured renal parenchyma l cells showed a stronger expression then in cells in vivo, but CD26 could not be found. In renal cell cancer (mixed clear cell/chromophilic, poorly d ifferentiated and clear cell type, moderately differentiated) CD13 and CD26 were labelled as in benign renal tissue, but CD26 appeared overexpressed. On the renal carcinoma cells Caki-1 and Caki-2, only one of the two peptida ses could be found. CD13 was present non-homogeneously in Caki-1, where the enzyme appeared to form clusters. When CD26 on the cultured renal carcinom a cells Caki-2, is compared with renal proximal tubular cells and renal car cinoma cells in tissue sections, a reduced expression is observed. CD13 was not detected in Caki-2, and CD26 was not found in Caki-1. These small chan ges on the cell surfaces can only be detected by electronmicroscopic method s. The differences in the distribution of APN/CD13 and DPP IV/CD26 in norma l and malignant cells are discussed in connection with literature. Further investigations, especially labelling studies on other neoplastic tissues an d cells, will be necessary in order to explain the precise role these membr ane peptidases in malignancies.